This invention relates to image presentation systems of the type employing a cathode-ray tube containing indexing elements for producing index signals indicative of cathode-ray beam-position, and especially to apparatus for maintaining the cathode-ray beam at an intensity sufficient to insure production of indexing signals of adequate strength without producing minimum beam intensities greater than are necessary for this purpose, and to apparatus for controlling the timing of blanking and pedestal pulses preferably utilized in such a system. More particularly, it relates in its preferred forms to apparatus for maintaining the minimum intensity of the cathode-ray beam in a color television system of the indexing type at a level which will provide the blackest possible appearance of those areas of the image intended to be substantially black or completely black while at the same time assuring adequate index signal. The invention also relates, in one respect, to apparatus for initiating horizontal blanking as soon as the beam leaves the image-presentation portion of the tube containing the indexing element and for providing a fixed pedestal of predetermined magnitude and controlled duration at the beginning of each horizontal line so as to provide suitable and reliable generation of index starting signals from starting index elements positioned near the beginning of each horizontal scan, and for providing other timing functions.
Image presentation systems are known in the prior art in which a cathode-ray tube is provided with an image display area on which an image is reproduced in response to repetitive horizontal scanning thereof by the cathode-ray beam, the intensity of the beam being controlled during its scanning so as to effect the desired image reproduction, and in which beam-position indicating index elements are located within the tube and within the area in which the image is reproduced so that, as the beam scans across the indexing elements, these elements produce an index signal representative of the instantaneous position of the beam. An example of such system, with particular reference to which the present invention will be described, comprises a color television image presentation system in which the image-reproduction area of the cathode-ray tube is covered with vertically-extending groups, typically triplets, of lines or stripes responsive to impingement by the beam to produce light of different colors. Each of the lines of each group, or triplet, produces a different color of light (typically red, green and blue) in response to beam impingement, and the groups or triplets are preferably substantially identical with each other and extend laterally adjacent each other across the entire inside face of the tube. The indexing elements, which may be light emitting indexing elements for example, are positioned in predetermined geometric relation to the sets of viewing phosphor lines, preferably so as to be scanned by the beam at a rate which is in odd integral half-multiple of the rate at which the triplets are scanned; for example, the index elements may be scanned at 3/2 the rate at which triplets are scanned.
Systems of the class identified above are described and claimed in my issued U.S. Pat. No. 3,013,113, issued Dec. 12, 1961; in my U.S. Pat. No. 2,892,123, issued June 23, 1959; in my U.S. Pat. No. 3,305,788, issued Feb. 21, 1967; in my co-pending application Ser. No. 132,692, filed Apr. 9, 1971.
In such systems, the indexing signals derived are normally processed and combined with received color television chroma signals in an appropriate manner such that the resultant signal used to control the chroma of the reproduced image recurs at the same rate as the rate at which the triplets are scanned, thereby in effect synchronizing the received chroma-controlling information with the scanning of the cathode-ray tube beam across the phosphor elements. A signal at least approximately representative of the brightness of the image to be reproduced is also generally applied to the beam-intensity controlling means for the cathode-ray tube, so that the reproduced image will have both proper brightness and proper color.
In such systems, the strength of the index signals produced depends upon the intensity of the cathode-ray beam impinging the index elements. If the beam intensity during impingement of the index elements is too small, either there will be no indexing signal produced or else the indexing signal produced will be inadequate in strength to operate properly the circuits to which it is supplied. Since in most cases, for example in color television systems of the type referred to above, the signal applied to the beam-intensity controlling means of the cathode-ray tube varies widely as a function of picture content, some affirmative measure must be taken to prevent the beam intensity from decreasing below the point at which the index signal becomes inadequate. At the same time, if the minimum beam intensity is set too high, then the minimum illumination in the reproduced image will also be relatively high; in the case of a color television system, this will set a limit on the maximum black which can be represented, and portions of the image which are intended to be completely black will emit a predetermined amount of light determined by the minimum beam intensity. It is of course desired that the black portions of the picture be as black as possible, rather than somewhat grey as will be the case if the minimum beam intensity is too high.
In my above-identified U.S. Pat. No. 2,892,123, the minimum cathode-ray beam current is set by a signal clipper which removes all excursions of the brightness controlling signal extending beyond the clipping level in the direction to reduce the beam intensity. In my co-pending application Ser. No. 132,692, filed Apr. 9, 1971, the minimum beam intensity is set by the bias applied to a clipper diode in the maximum brightness circuit thereof, provided that the output of the clipper diode is applied through direct coupling to the beam-intensity controlling means of the cathode-ray tube; where there is AC coupling between the maximum brightness clipper and the beam-intensity controlling means of the cathode-ray tube, a DC restorer is utilized which, in effect, clamps or limits at a predetermined level the maximum excursion of the signal in the direction tending to cut off the cathode-ray beam.
While satisfactory for many purposes, these clipping and/or DC restoring circuits must in practice be offset somewhat from the conditions producing the minimum usable beam intensity, so as to accommodate unavoidable variations in circuit parameters. Thus the strength of the indexing signal produced can change because of line voltage variations, aging of parts of the circuit, changes of temperature of the circuit elements, etc. If the minimum beam intensity is initially set too close to the value at which the indexing signal becomes inadequate, such changes may cause it to fall below the minimum adequate level and the index signal may become inadequate for proper indexing. This can cause the reproduced image to become completely blank, or partially blank, or to contain "torn" horizontal lines. If one adjusts the minimum beam intensity so that, even when such changes in circuit conditions occur, the beam intensity will never be reduced below the minimum level required for proper indexing, then the minimum beam intensity will usually be greater than necessary, and the maximum blackness will not be as black as could otherwise have been provided, particularly when the unavoidable changes in circuit parameters occur in the direction to increase the cathode-ray beam intensity. Under such conditions, either the minimum beam intensity must be factory-set higher than would otherwise be desirable, or else the viewer will find it desirable to adjust the minimum beam intensity from time to time as conditions in the circuit change. Both of these possibilities are undesirable.
To some extent these problems can be overcome by brute-force designing using conventional techniques, such as a darker-than-conventional face plate for the cathode-ray tube, but this reduces maximum brightness, or leads to greater cost to recover the same maximum brightness; or by using an index pickup device which gathers more energy, or which has a better signal-to-noise ratio, but this too can become very expensive if carried out to a fully satisfactory extent.
Accordingly, it is an object of the invention to provide a new and useful electrical system for controlling the minimum beam intensity in an indexing type of cathode-ray tube.
Another object is to provide a system which will automatically maintain as black as possible the portions of the cathode-ray tube image intended to be black, without requiring readjustments of circuit controls by the operator.
Another object is to provide such a system which is not upset by signals generated when the scanning beam departs from the useful area containing the index elements.
A further object is to provide a timing system for controlling the horizontal blanking, and for controlling the application of a starting pedestal pulse to the beam-intensity controlling means of the cathode-ray tube at the beginning of each line, which is fully compatible with the automatic minimum beam-intensity control system referred to above.
A further object is to provide a system which maintains the desired minimum beam intensity despite variations in a large number of factors tending to influence minimum beam intensity.
A further object is to provide means which will operate effectively to properly limit maximum blackness despite variations of maximum blackness which might otherwise be caused by an automatic maximum brightness control operating to control average brightness; this objective including the more specific objective of making the present invention fully compatible with the system disclosed in my above-cited U.S. patent application Ser. No. 132,692, filed Apr. 9, 1971.
A further object is to provide, through relatively low cost, small-signal control circuits, a performance in regard to reproduction of dark areas of the reproduced image, which is comparable to that which could otherwise be reliably obtained only by relatively more costly means.